Starch, which is a mixture of amylose and amylopectin, is a glucan stored in cells of seed or rhizome of higher plants. Amylose is a α-1,4-glucan in which glucoses are linearly bound by α-1,4-glucosidic linkage. Amylopectin has branched structures in which some glucoses of other α-1,4-glucan are bound to branches of α-1,4-glucans with glucose polymerization degree of generally 6 or more by α-1,6-glucosidic linkage. Water dispersion of starch was swollen by heating to be gelatinized starch, but when it is left after cooling, it tend to easily gelate by retrogradation. Gelatinized starch has been used as food by since ancient times, as a main food material with good workability, low cost, and good storage quality, and also as a thickener, water-retention agent, or colloid-stabilizing agent for improving food properties or reserving food qualities. Liquefied starch is used for an industrial material for production of glucose, isomerized sugar, maltooligosaccharides, and syrups. But gelatinized or liquefied starch have disadvantageous properties as easily gelating by retrogradation during storage resulting in unfitting for food with hardening by loss of water-retentivity and being difficult to be act on by a enzyme.
Under these circumstances, many attempts were made to give retrogradation-resistance to gelatinized starch by chemical modification, such as esterification, etherification, or oxidation of starch. But modified starch in which glucose lost its natural structure by addition of particular kind of functional groups is not commonly used because of safety concern about long-term intake of them.
A method of adding retrogradation-resistance to starch by enzymatic modifications is proposed. Japanese Patent Kokai 134104/1996 proposed a method of forming water-soluble macrocyclic glucans by allowing branching enzyme (EC 2.4.1.18), which cleave .alpha.-1,4-glucosidic linkages and form .alpha.-1,6-glucosidic linkages by transglycosilation, 4-.alpha.-glucanogransferase (D-enzyme, EC 2.4.1.25) or CGTase (EC 2.4.1.19) to act on starch liquefying solution. But because the forming of macrocyclic glucans accompanies large decrease of molecular weight of substrate resulting in low viscosity, there is a problem that the characteristics of the material liquefied starch is lost. Japanese Patent Kokai 294601/2001 proposed the method of forming highly branched starch, mainly constructed of branches with glucose polymerization degree of 4 to 7 and densely-branched compared to the material starch, using the branching enzyme from Neurospora crassa which hardly lower the molecular weight of the starch. But above branched starch is not yet led to industrial preparation and application because the safety of Neurospora crassa, an atypical fungus, is not determined. Japanese Patent Kokai 78497/2002 proposed a method of forming branched starch mainly constructed of branches with DP of glucose of 6 or 7, in which SBE-II (branching enzyme from barley) and phosphorylase are allowed to act on glucose-1-phosphate and maltoologosaccharides. But it is very difficult to industrially use SBE-II (branching enzyme from barley) phosphorylase and glucose-1-phosphate. Under above circumstances, providing a starchy substance is expected, in which properties including retrogradation property were improved with little decrease of molecular weight.